It is known that stratospheric balloons with a variable volume may present very significant volumes (greater than 10.sup.6 m.sup.3) and carry heavy loads up to altitudes on the order of 40 to 45 km. These stratospheric balloons are characterized especially in that they operate with a lighter than air gas from which they draw their ascending force, and in that the volume at a state of fullness is much greater (of a factor much greater than 10) than their volume at their bulbous state (a state of reduced inflation). This type of balloon is, for example, described in U.S. Pat. No. 3,312,427. These stratospheric balloons differ completely from hot air balloons which are balloons having a constant volume, using the gas of the atmosphere in which they are immersed and drawing their ascending force, either from a burner or from ambiant radiation (solar or infrared). Solar or infrared hot air balloons are produced in such a manner as to captivate permanently the maximum solar or infrared energy and, for this reason, are provided on at least half of their surface with a device for trapping solar or infrared radiation. The hot air balloons may for example be illustrated by French patent No. 2,418,150 or U.S. Pat. No. 4,262,864.
The basic problem which is presented by variable volume stratospheric balloons, which are the subject of this invention, resides in their very short length of life, on the order of 24 hours, and which may only be extended by several days by releasing significant quantities of ballast. In effect, during the daytime period, when a stratospheric balloon reaches a state of fullness, a portion of the gas is expelled until the rising force is annulled, the balloon being stabilized at its flight ceiling. At night, the gasses of the balloon are subjected to a significant cooling which causes a considerably diminution of the volume thereof and an irreversible descent. Only a releasing of ballast then permits a stabilization (on the order of 10% of the weight of the assembly). However, the rising force which is then restored, appears the following day and results in a new expulsion of the gas at the ceiling, such that the operation of the release of the ballast must be done again each night in order to avoid an irreversible descent. The quantities of ballast released being limited, the duration of life of such balloons cannot generally exceed 4 or 5 days. This limitation of the period of life is a serious drawback of this type of balloon, while the obligation for releasing ballast of sizeable quantity is very penalizing since it requires, for a given useful load, increasing the volume of the balloon and the mass of aerostatic gas, and as a consequence, leads to an increase in the cost of a flight.
The present invention proposes to overcome the drawbacks mentioned above for stratospheric balloons of the variable volume, open type.
The essential object of the invention is to avoid, by means other than a release of ballast, an irreversible descent of the balloon during the nocturnal period, in such a manner as to assure a stabilization of the balloon between the upper levels (daytime periods) and the lower levels (nocturnal periods) during a plurality of daytime and nighttime cycles.